This invention relates to ultrasonic alarm devices which detect the presence or absence of a moving object by utilizing the Doppler effect, to provivde an alarm signal, and more particularly to an ultrasonic alarm device which is used as a burglary preventing device.
It goes without saying that an ultrasonic alarm device must be able to detect an intruder entering the watching area, by utilizing a reflected ultrasonic wave, to provide an alarm signal, and it is essential that the device will not operate errroneously when an external disturbance similar to a reflection signal due to the movement of an intruder (hereinafter referred to as "the movement of a person") takes place in the watching area.
The frequency components of a Doppler signal due to the movement of a person are of the order of 80 Hz when he walks at a speed of about 0.5 m/s, and frequency components due to motion of his hands and feets are distributed on both sides of 80 Hz.
A first example of an external disturbance other than the Doppler signal due to the movement of a person is the wind from a cooler or a draft which is equal to or lower than the Doppler signal due to the movement of a person in frequency. The frequency distribution is such that, in the frequency range of 0 to 150 Hz, the amplitude is decreased as the frequency is increased.
A second example of an external disturbance is for instance a continuous noise signal produced by a telephone bell, the frequency components of which distribute over a relatively wide frequency range including the frequency band of the Doppler signal due to the movement of a person. This external disturbance has a substantially constant amplitude in a frequency range of zero to several kilo-Hertz (KHz) or more.
A third example of an external disturbance is an instantaneous noise such as a noise generated when an object falls down, or an electric click sound. The frequency distribution is similar to that of a continuous noise, except that the noise generation time is short.
A fourth external disturbance is a Doppler signal due to motion of a small animal such as a bird or an insect. The frequency components are substantially similar to those of the Doppler signal due to motion of a person. However, the central frequency is abruptly shifted around the frequency band of the Doppler signal due to motion of a person, because the small animal moves around in different directions and at different speeds.
A countermeasure against each of these external disturbances has been proposed in the art; however, a counter measure against all of the external disturbances has not been proposed yet.